Southern Antarctic Peninsula Research Articles

Provenance of Triassic-Cretaceous Sandstones in the Antarctic Peninsula: Implications for Terrane Models During Gondwana Breakup

ABSTRACT The Antarctic Peninsula is the largest of twelve micro-continental fragments between southern South America, East Antarctica, and New Zealand. Their original positions and movement history following Gondwana breakup are poorly constrained. Recent work on the Antarctic Peninsula fore-arc/arc/back-arc suggests it consists of two suspect terranes that originated elsewhere and collided with the Pacific margin of Gondwana in the Cretaceous. This study reviews provenance models for three laterally extensive, kilometers-thick sedimentary units in the Antarctic Peninsula using previous studies, and new sandstone detrital modes and chemistry. 1) The Trinity Peninsula Group accretionary complex (Triassic) was possibly derived from a glaciated continental margin and may consist of several accreted tracts. The deformed, quartzose View Point and Legoupil formations were eroded from a deeply eroded, stable continental margin containing an old arc and mature quartzose sandstones, with provenance ages suggesting sources in north Patagonia and local Antarctic Peninsula basement. The weakly deformed and more feldspathic Hope Bay Formation was derived from an eroded arc, possibly the early Permian arc of the North Patagonian massif. 2) The LeMay Group accretionary complex (middle Triassic-middle Cretaceous) is dominated by arkoses derived from an eroded continental margin of Carboniferous and Permian age, possibly in the Marie Byrd Land sector, with several belts of volcanilithic sandstones representing episodes of arc magmatism. The youngest part of the LeMay Group contains abundant metamorphic detritus derived from basement uplifted during mid-Cretaceous terrane docking. 3) The Latady Formation in the southern Antarctic Peninsula has a completely different petrology, comprising mature sublitharenites ( 190 Ma) of Gondwanan provenance, deposited in restricted rift basins. Younger volcanic litharenites (180 Ma) are dominated by REE-enriched low Eu/Eu* rhyolitic material, eroded from silicic calderas associated with Gondwana breakup. Abundant dacitic to andesitic detritus in the youngest LF (160-145 Ma) was derived from a contemporaneous arc, possibly on the Thurston Island block, as the Latady evolved into a back-arc basin.

Tectonic evolution of the Pacific margin of Antarctica 1. Late Cretaceous tectonic reconstructions

We present new Late Cretaceous tectonic reconstructions of the Pacific margin of Antarctica based on constraints from marine magnetic data and regional free‐air gravity fields. Results from interpretation of new seismic reflection and gravity profiles collected in the Bellingshausen Sea are also incorporated in the reconstructions. The reconstructions show regional constraints on tectonic evolution of the Bellingshausen and Amundsen Seas following the breakup between New Zealand and West Antarctica. The breakup began at c. 90 Ma with the separation of Chatham Rise, probably accompanied by the opening of the Bounty Trough. Campbell Plateau separated from West Antarctica later, during chron 33r (83.0–79.1 Ma). A free‐air gravity lineation northeast of Chatham Rise represents the trace of a triple junction that formed as a result of fragmentation of the Phoenix plate a few million years before Chatham Rise separated from West Antarctica. Remnants of the western fragment, the Charcot plate, are preserved in the Bellingshausen Sea. Subduction of the Charcot plate stopped before 83 Ma, and part of it became coupled to the Antarctic Peninsula across the stalled subduction zone. Subsequent convergence at the western margin of this captured ocean floor produced the structures that are the main cause of the Bellingshausen gravity anomaly. Part of a spreading ridge at the western boundary of the Phoenix plate (initially Charcot–Phoenix, evolving into Marie Byrd Land–Phoenix, and eventually Bellingshausen–Phoenix (BEL–PHO)) probably subducted obliquely beneath the southern Antarctic Peninsula during the Late Cretaceous. All of the Phoenix plate ocean floor subducted at the Antarctic Peninsula margin during the Late Cretaceous was probably <14 Myr old when it reached the trench. Several observations suggest that independent Bellingshausen plate motion began near the end of chron 33n (73.6 Ma). Reconstructions in which part of the West Antarctic continental margin, including Thurston Island, is assumed to have been within the Bellingshausen plate seem more plausible than ones in which the plate is assumed to have been entirely oceanic.

Mid-Cretaceous ductile deformation on the Eastern Palmer Land Shear Zone, Antarctica, and implications for timing of Mesozoic terrane collision

Ar–Ar dating of high-strain ductile mylonites of the Eastern Palmer Land Shear Zone in the southern Antarctic Peninsula indicates that reverse movement on the shear zone occurred in late Early Cretaceous times (Albian), and not latest Jurassic times as previously supposed. The Eastern Palmer Land Shear Zone forms a major tectonic boundary, separating suspect arc terranes from rocks of Gondwana continental affinity. The dated mylonites are developed in Lower Jurassic plutonic rocks at Mount Sullivan, eastern Palmer Land, and form part of a zone of ductile reverse deformation up to 25 km wide. Biotite from a fine-grained mafic mylonite yields an Ar–Ar cooling age of 102.8±3.3 Ma. Movement of this age on the Eastern Palmer Land Shear Zone is coeval with circum-Pacific deformation, possibly related to a mantle superplume event, and provides support for allochthonous-terrane models for the Antarctic Peninsula with accretion in post-Early Cretaceous times.

A mid-Cretaceous age for the Palmer Land event, Antarctic Peninsula: implications for terrane accretion timing and Gondwana palaeolatitudes

New structural and age data suggest that West Gondwana may have been at lower palaeolatitudes than previously interpreted from Albian sequences in Gondwana marginal suspect terranes. The Palmer Land event, which juxtaposed Mesozoic terranes on the Gondwana margin, deformed granitoids in the southern Antarctic Peninsula. U–Pb SHRIMP dating of zircons from a microgranite dyke yields a crystallization age of 106.9± 1.1 Ma. This result and re-interpretation of the structural position of another granite pluton date the Palmer Land event, and probable terrane collision, as late Early Cretaceous, and not latest Jurassic as formerly interpreted.

Detecting rift basins in the Evans Ice Stream region of West Antarctica using airborne gravity data

A total of 11,500 line km of aerogravity data have been used to construct an free-air gravity anomaly map for the Antarctic region that may contain the microplate boundary between the Haag Nunataks block and southern Antarctic Peninsula. Along-line free-air gravity anomaly data resolved wavelengths of 9 km or greater with better than 5 mGal accuracy. Coincident radio echo soundings provided data to construct a digital terrain model. The gravity effect of the terrain was calculated by Gauss–Legendre quadrature (GLQ) and spectrally correlated with the free-air gravity data. Terrain-correlated free-air anomalies related to possible isostatic imbalances of the crust were separated from terrain-decorrelated anomalies that may reflect intra-crustal density contrasts. Subtracting terrain-correlated free-air anomalies from the gravity effects of the terrain yielded compensated terrain gravity effects (CTGE) that were used to model the Moho by inversion. The results indicate moderate but significant crustal thinning below the Evans Ice Stream that is consistent with an extensional origin for the deep, wide, steep-sided trough that contains the ice stream as well as the continued elevation of the footwall flank of the basin. Changes along the axis of the rift, both in the gravity anomaly field and the distribution of Moho topography, can be explained by processes associated with continental lithospheric extension. Subsequently, many of the features produced by extension have been modified by glacial erosion and the sub-ice topography and gravity data reflect this.

Origins of Large Volume Rhyolitic Volcanism in the Antarctic Peninsula and Patagonia by Crustal Melting

Voluminous rhyolitic volcanism along the palaeo-Pacific margin of Gondwana was marked by three principal episodes of magmatism. The first of these ( V1) is essentially coincident with the main episode of Karoo–Ferrar magmatism at ∼184 Ma. A younger ( V2) episode occurred at ∼168 Ma, and a third episode ( V3) occurred in the interval 157–153 Ma. We evaluate the origin of V1 and V2 rhyolites from the Antarctic Peninsula using major and trace element and isotopic (Sr, Nd, O) data. An isotopically uniform (87Sr/86Sri ∼0·707; εNdi ∼ −3) andesite–dacite magma was generated as a result of anatexis of ‘Grenvillian age’ hydrous mafic lower crust, linked to earlier, arc-related underplating. The lower-crustal partial melts would have mixed with fractionated components of the mafic underplate, followed by subsequent storage and homogenization. Early Jurassic ( V1) rocks of the southern Antarctic Peninsula are interpreted as melts of upper-crustal paragneiss, which have mixed with the isotopically uniform magma in upper-crustal magma chambers. The V2 rhyolites are the result of assimilation–fractional crystallization of the isotopically uniform magma. This occurred in upper-crustal magma chambers involving assimilants with similar isotopic composition to that of the magma. A continental margin setting was crucial in developing hydrous, readily fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the Discovery–Shona–Bouvet group of plumes, thought to be responsible for the Karoo magmatic province. The progression (old to young) of volcanism from NE to SW in Patagonia and south to north in the Antarctic Peninsula is consistent with migration away from the mantle plumes towards the proto-Pacific margin of Gondwana during rifting and break-up.

The eastern Palmer Land shear zone: a new terrane accretion model for the Mesozoic development of the Antarctic Peninsula

A major ductile fault zone, the eastern Palmer Land shear zone, has been identified east of the spine of the southern Antarctic Peninsula. This shear zone separates newly identified geological domains, and indicates that during Late Jurassic terrane accretion and collision, two and possibly three separate terranes collided, resulting in the Palmer Land orogeny. The orogeny is best developed in eastern Palmer Land and eastern Ellsworth Land. There, shallow-marine sedimentary rocks of the Latady Formation, and a metamorphic and igneous basement complex of possible Lower Palaeozoic to pre-Early Jurassic age, are thrust and folded. This forms an arcuate, east-directed, foreland, fold and thrust belt up to 100 km wide and 750 km long, parallel to the axis of the Antarctic Peninsula. The newly identified Antarctic Peninsula domains include: (1) a parautochthonous Eastern Domain that represents part of the margin of the Gondwana continent, comparable to the Western Province of New Zealand, the Ross Province superterrane of Marie Byrd Land, the Eastern Series of south-central Chile, the Pampa de Agnía and Tepuel rocks of north Patagonia, and the Cordillera Darwin rocks of Tierra del Fuego, (2) a suspect Central Domain that represents an allochthonous, microcontinental, magmatic arc terrane, comparable to the Median Tectonic Zone of New Zealand, the Amundsen Province superterrane of Marie Byrd Land, and Coastal Cordillera of north Chile and (3) a suspect Western Domain, with strong similarities to the Eastern Province of New Zealand, Western Series of south-central Chile, and Chonos metamorphic complex of north Patagonia, that represents either a subduction–accretion complex to the Central Domain, or another separate crustal fragment. Although an allochthonous terrane hypothesis for the Antarctic Peninsula remains to be fully tested, this has much in common with models for the New Zealand and South American parts of the Pacific margin of Gondwana. The identification of a potential allochthonous terrane–continent collision zone allows us to define the edge of the Gondwana continent in the Antarctic Peninsula sector of the supercontinent margin, which has implications for Mesozoic reconstructions of Gondwana.

Sedimentary processes across the continental rise of the southern Antarctic Peninsula

A series of large sediment mounds have been identified along the Pacific portion of the Antarctic Peninsula continental rise. These mounds are composed of sediment delivered to the continental rise during the advance and retreat of grounded ice across the shelf. The stratigraphic development of one of these sediment deposits, the Tula sediment mound, is examined to investigate how the onset of glaciation influenced the deep-sea depositional environment along this portion of the margin. The stratal relationships, associated facies distribution, and the physiography observed along the southern Antarctic margin reflect the waxing and waning of the Antarctic ice sheets; various processes erode, transport, and deposit sediment along the outer shelf, slope, and rise throughout a glacial cycle. A deep-sea erosional unconformity is apparent at the base of the Tula deposit. This surface may reflect the first onset of intensified bottom water circulation along the margin perhaps induced by the tectonic opening of the Drake Passage. The Tula sediment mound is comprised predominantly of canyon/overbank systems. Evidence for the onset of canyon cutting and the development of the thick overbank deposits (> 2 km) is found immediately above the deep-sea erosional unconformity. Relating increased canyon cutting across the continental rise to the fluctuation of ice across the shelf implies that the onset of predominantly glacial conditions commenced soon after the onset of intensified bottom water circulation along this margin. The volume of sediment associated with the Tula deposit and the apparent ‘point source’ distribution of the channel systems suggests that much sediment was transported to the slope and rise perhaps by meltwater processes through these canyons.

The age and stratigraphy of fore-arc magmatism on Alexander Island, Antarctica

Fore-arc magmatic sequences associated with high Mg number andesite lavas unconformably overlie LeMay Group accretionary complex in Alexander Island. High-resolution 40Ar/39Ar, U–Pb zircon, fission track and K–Ar ages demonstrate that subduction-related fore-arc magmatism migrated northwards along the length of Alexander Island between c. 80 Ma and c. 46 Ma. The magmatic rocks represent a third of the western margin of the Antarctic Peninsula magmatic arc and are critical to the understanding of the final phase of subduction along the southern Antarctic Peninsula margin. The onset of late Cretaceous magmatism is recorded by poorly exposed volcanic rocks on Monteverdi Peninsula (79.7±2.5 Ma). In central and northern Alexander Island, the magmatic rocks can be distinguished by the proportion, range and types of lithofacies present, and by the periods of magmatism represented. The volcanic rocks of the Colbert Mountains range in age from c. 69–62 Ma and are dominated by large volume dacitic and rhyolitic crystal-rich ignimbrites interpreted as caldera-fill deposits. Elgar Uplands sequences range in age from c. 55–50 Ma, and contain approximately equal proportions of pyroclastic deposits and less evolved (basaltic-andesite and andesite) lavas including high Mg number andesite lavas near the base of three sequences. The volcanic rocks of Finlandia Foothills probably represent the youngest calc-alkaline units on Alexander Island (48±2 Ma). The sequence is lithologically similar to the Elgar Uplands and also contains high Mg number andesite lavas, but it is dominated by polymict conglomerates, with minor lavas, which were deposited in a graben associated with regional extension. Plutonic rocks exposed in the Rouen Mountains, adjacent to the Elgar Uplands, yielded a U–Pb age of 56±3 Ma which is in discordance with a previously published Rb–Sr age (46±3 Ma), probably due to hydrothermal perturbation of the Rb–Sr system. Northwards migration of magmatism was caused by the progressive collision and subduction of three ridge segments prior to the previously reported ridge crest–trench collisions that occurred c. 20–30 Ma later and following which subduction ceased.

Technical procedures for aeromagnetic surveys in Antarctica during the Italian expeditions (1988-1992)

For most of Antarctica, the geophysical data now available are those of aeromagnetic surveys performed there from 1950 to 1960. Until 1984, the inaccurate positioning and insufficient monitoring of geomagnetic time variations allowed the investigation of the geomagnetic residual field only along profiles. The Ganovex IV aeromagnetic survey, performed by BGR-USGS over the Ross Sea and the Northern Victoria Land, and the geophysical investigations of BAS on the Southern Antarctic peninsula and the Ronne ice shelf region corresponds to the recent advancement of these techniques in Antarctica. The first experiments of aeromagnetic measurements, during the Italian expeditions in Antarctica were made during the 1988-1989 field season. Some geomagnetic helicopter borne profiles were accomplished with a Proton Precession Magnetometer (PPM) in the Terra Nova Bay-Gerlache Inlet area. In the 1989-1990 ItaliAntartide expedition some profiles were flown over the suture between the Wilson and Bower terranes, in Northern Victoria Land. During the 1991-1992 expedition, in cooperation with researchers of BGR (Bundesanstalt fur Geowissenschaften und Rohstoffe) of Hannover, the GITARA I (German ITalian Aeromagnetic Rescarch Antarctica) program, as part of the LIRA (Litospheric Investigation in the Ross Sea Area) project, was carried out. The investigated area lies between the latitudes 74°18' S and 75°18' S and the longitudes 160°30' E and 164°30'E and it corresponds to a portion of the North Victoria Land, located between the Eisenhower Range and the, DrygaIski Ice Tongue. The survey was made with a Cesium vapour magnetometer. The positioning system was of the “Range-Range” type, it consisted of three transmitters (beacons), installed inside the investigated area and located with GPS measurements. The line spacing was 4.4 km, with tie lines every 22 km. The survey covered an area of 6500 km2 . Four PPM base stations for the determination of the time variation corrections were installed. The aeromagnetic anomalies wilI allow the comparison with the ground geomagnetic surveys performed during the previous Antarctic expeditions, also in connection with the aeromagnetic maps of Ganovex IV, located at the northern and eastern boundaries of the GITARA area.

New paleomagnetic data from the Antarctic Peninsula and their tectonic implications

New paleomagnetic data presented here further constrain the relative motion of the Antarctic Peninsula relative to East Antarctica during the Mesozoic development of the southern ocean basins. The Antarctic Peninsula (AP) is one of four crustal blocks that define West Antarctica, the others being the Ellsworth‐Whitmore Mountains (EWM), Thurston Island‐Eights Coast (TI), and Marie Byrd Land (MBL). A Jurassic pole (∼155Ma) (124°E, 64°S, A95=7.1°, N=10 VGPs) was obtained from the AP block which suggests that the AP block rotated clockwise between ∼175 and ∼155 Ma due to significant early opening in the Weddell Sea basin. A new Early Cretaceous paleomagnetic pole (182°E, 74S, A95=5.9°, N=6 site mean VGPs) indicates that the AP block was in or near to its present‐day position with respect to East Antarctica by ∼130 Ma. Between ∼155 and 130 Ma, counterclockwise rotation of the AP‐TI blocks, together with the southward motion of East Gondwanaland, probably resulted in subduction of Weddell Sea ocean floor beneath the southern AP block and initiated the Palmer Land deformational event. A ∼130 Ma pole from the TI block requires clockwise rotation of the TI and possibly the EWM blocks between 130 and ∼110 Ma producing sinsistral strike‐slip motion between the EWM block and East Antarctica and dextral transpressional motion between the TI‐EWM blocks and the AP block. New AP block ∼110 Ma and ∼85 Ma poles from this study (199°E, 74°S, A95=6.9°, N=13 VGPs; 152°E, 86°S, A95 = 7.5°, N=6 VGPs, respectively) are similar to equivalent age poles from East Antarctica and suggest little or no relative motion between the Antarctic Peninsula and East Antarctica. Northern and southern Antarctic Peninsula mid‐Cretaceous poles are very much alike suggesting that the “S” shape of the Antarctic Peninsula is not due to oroclinal bending since ∼110 Ma.

Creation and destruction of Weddell Sea floor in the Jurassic

New paleomagnetic data invite reassessment of the breakup history of parts of the Pacific margin of Gondwana due to the Mesozoic opening of the southern ocean basins. These data indicate clockwise rotation of the Antarctic Peninsula relative to East Antarctica between ∼175 and ∼155 Ma that would have created as much as 1000 km of pre-anomaly M25 ocean floor in the Weddell Sea basin. The paleomagnetic data also suggest that between ∼155 and ∼130 Ma, the Antarctic Peninsula rotated counterclockwise relative to East Antarctica, rotation that, along with the southward motion of East Antarctica, would have caused subduction of the newly created Weddell Sea ocean floor beneath the southern Antarctic Peninsula. East-vergent folding and thrusting of Jurassic back-arc basin rocks along the southeastern Antarctic Peninsula, known as the Palmer Land event, probably resulted from deformation along this newly created convergent margin. The presence of Jurassic Weddell Sea floor would have isolated eastern Gondwana (East Antarctica, Australia, and India) from the Antarctic Peninsula and western Gondwana (Africa and South America), encouraging increased marine faunal connections between the Pacific and Weddell Sea- Mozambique-Somali ocean basins during the ∼175 to ∼130 Ma interval. After ∼130 Ma, the Antarctic Peninsula would have been connected to East Antarctica via the other West Antarctic terranes, thereby facilitating land migrations between eastern and western Gondwana.

New paleomagnetic data from Thurston Island: Implications for the tectonics of west Antarctica and Weddell Sea opening

Paleomagnetic data from three West Antarctic crustal blocks (Antarctic Peninsula (AP), Thurston Island‐Eights Coast (TI), and the Ellsworth‐Whitmore Mountains (EWM) indicate that there has been motion between the individual blocks and motion relative to East Antarctica during the Mesozoic. A Triassic paleomagnetic pole from the TI block (116°E, 61°S, A95 = 19.4°, N = 3 VGPs) appears to indicate that the block has rotated ∼90° relative to East Antarctica between 230 Ma and 110 Ma. Our previously reported Middle Jurassic paleomagnetic pole from the EWM block indicates that a 90° rotation relative to East Antarctica occurred sometime between the Cambrian and 175 Ma. We believe that the 90° counterclockwise EWM rotation occurred between ∼220 Ma and 175 Ma related to the development of post‐Gondwanide Orogeny shear zones. The motion of the AP, TI, and EWM blocks appears to be linked during the mid‐to late Mesozoic to three major events in the evolution of the southern ocean basins. Opening in the Mozambique‐Somali‐Weddell Sea basins may have produced major counterclockwise rotation of the TI block with respect to East Antarctica between the Jurassic and Early Cretaceous based on new Late Jurassic (145°E, 64.5°S, A95 = 7°,N = 5 VGPs) poles. We believe that the TI rotation, as well as deformation in the southern AP block, was caused by collision and shearing of the EWM block against the other two as the EWM block moved southward with East Antarctica. An Early Cretaceous paleomagnetic pole (232°E, 49°S, A95 = 7.9°, N = 5 VGPs) from the TI block requires that between the Early and mid‐Cretaceous there was clockwise rotation, with respect to East Antarctica, of the AP‐TI‐EWM blocks (an entity we call Weddellia). A change in the opening history of the Weddell Sea basin caused by initiation of spreading in the South Atlantic ocean basin at ∼130 Ma probably started Weddellia's clockwise rotation. Two new ∼110 and ∼90 Ma poles from the TI block (210°E, 73°S, A95 = 7.6°,N = 7 VGPs and 161°E, 81°S, A95 = 3.9°,N = 18 VGPs, respectively) are similar to equivalent age poles from the AP block and East Antarctica and indicate that Weddellia was at or near its present‐day position with respect to East Antarctica by ∼110 Ma. This corresponds to a time of major plate reorganization in the South Atlantic and southeast Indian Oceans. Based on both the new TI paleomagnetic data and previously reported data from Marie Byrd Land (MBL), dextral shearing would be expected to have occurred between MBL and Weddellia since the mid‐Cretaceous. Pine Island Bay, the area between the TI and MBL blocks, marks a fundamental and complex tectonic boundary in West Antarctica that we propose has largely been a zone of transcurrent shearing.

Reconnaissance geochemistry of the Lassiter Coast Intrusive Suite, southern Antarctic Peninsula

Calc-alkaline plutonic rocks of the Lower Cretaceous Lassiter Coast Intrusive Suite are the predominant rocks exposed in the Lassiter, Orville, and southern Black Coasts and eastern Ellsworth Land of West Antarctica. Compositions range from granite to gabbro. K-Ar and Rb-Sr isotopic ages are in the range 130–95 m.y., Early Cretaceous. Magmas were formed in the roots of an Andean-type magmatic arc developed on continental crust during Pacific plate subduction and were emplaced at high crustal levels as one-mica (biotite) granitoids. Our data do not demonstrate transverse asymmetric geochemical variation across the southern Antarctic Peninsula batholith complex in either the southern Black Coast or Lassiter Coast. We have, however, few samples from the eastern half of the batholith in this area. In eastern Ellsworth Land, however, where our samples span the entire width of the batholith, K 2 O index [1,000 K 2 O/(SiO 2 -45)] increases to the south away from the site of the former subduction zone. The batholithic rocks of the southern Antarctic Peninsula are similar in form, regional extent, composite nature, over-all composition, and geochemistry to the plutonic rocks of the northern Antarctic Peninsula, to southeastern Australian I-type granitoids, and to other circum-Pacific batholith terrenes. Geochemical features of the southern Antarctic Peninsula batholith complex that are shared with other circum-Pacific subduction-related magmatic arcs include moderate to low iron enrichment, late K 2 O enrichment, transition from diopside-normative to corundum-normative rocks in the range 63% to 69% SiO 2 , high levels of large-ion lithophile elements (Ba, K, LREE, Rb, Sr, Th), high Ba/Sr, high large ion lithophile element/high field strength element ratios, and low K/Rb ratios. Chondrite-normalized, rare-earth–element trends in the southern Antarctic Peninsula are similar to those of many western North and South American batholith complexes. Light rare-earth elements are only slightly enriched over heavy rare-earth elements in the most mafic samples, increase in abundance (to as much as 160 times chondrite values) and are more strongly enriched in intermediate and felsic rocks, but decline slightly in abundance in the most felsic compositions. Europium anomalies are negligible in the most mafic rocks but become moderately negative in intermediate and felsic samples. Variation in mineral assemblages is controlled by fractionation of clinopyroxene and plagioclase in the most mafic rocks, by hornblende and plagioclase in intermediate and felsic rocks, and by plagioclase and rare-earth-element–rich accessory minerals in the most felsic rocks.

Cretaceous and Early Tertiary paleomagnetic results from the Antarctic Peninsula

Paleomagnetic results are presented for early Tertiary and Cretaceous igneous and volcanic units from the northern Antarctic Peninsula. When combined with previously published data, they permit the construction of a polar wander path that is very similar to one derived for the East Antarctic craton by rotating Tertiary and Cretaceous poles from India and Australia into the Antarctic frame of reference. When considered in this context, the paleomagnetic data from the Orville Coast, which was previously interpreted as evidence in favor of oroclinal bending or relative rotation of the southern Antarctic Peninsula with respect to the northern part, may be considered as an in situ resulte The implications of the new data are as follows: (1) The Antarctic Peninsula down to the Orville Coast has been a part of the Antarctic plate since 100 m.y. ago. (2) Movement of the Antarctic Peninsula required to avoid overlap in reconstructions of Gondwanaland must have occurred before 100 m.y. ago.

Evaporite encrustations and sulphide oxidation products from the southern Antarctic Peninsula

Abstract White evaporite encrustations collected from 66 sites scattered over 30 000 km2 of southern Antarctic Peninsula are dominantly gypsum and calcite, and subordinate aragonite and thenardite. Natrojarosite occurs as locally developed yellow surface stains on approximately 30% of the white encrustations examined. Water, which forms by the melting of snow blown into cracks in highly fractured rocks by ground blizzards, dissolves soluble ions from minerals in these rocks as it percolates into them. The white encrustations appear to be deposited when this water later rises to the surface by capillary action and evaporates. Yellow surficial salts (alunite, fibroferrite, natrojarosite) collected at 5 sites form in place by the near-surface oxidation of pyrite disseminated in the sedimentary and plutonic rocks. Green surficial salts (atacamite, antlerite, brochantite, plancheite) collected at 4 sites form in place by the near-surface oxidation of copper sulphides under conditions of low acidity. This is th...

Paleomagnetic evidence for oroclinal bending of the southern Antarctic Peninsula

Research Article| July 01, 1980 Paleomagnetic evidence for oroclinal bending of the southern Antarctic Peninsula KARL S. KELLOGG KARL S. KELLOGG 1U.S. Geological Survey, Box 25046, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information KARL S. KELLOGG 1U.S. Geological Survey, Box 25046, Federal Center, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1980) 91 (7): 414–420. https://doi.org/10.1130/0016-7606(1980)91<414:PEFOBO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation KARL S. KELLOGG; Paleomagnetic evidence for oroclinal bending of the southern Antarctic Peninsula. GSA Bulletin 1980;; 91 (7): 414–420. doi: https://doi.org/10.1130/0016-7606(1980)91<414:PEFOBO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Paleomagnetic results from the investigation of 13 magnetically stable units (92 oriented rock samples) of Upper Cretaceous (“Andean”) plutons and dikes from the Orville Coast and eastern Ellsworth Land, Antarctica, define a mean direction of magnetization of I = −77°, D = 51° (α95 = 5.9°), with a paleomagnetic pole at 71°S, 165°W. The sampled units were emplaced after the Late Jurassic to Early Cretaceous intense folding associated with subduction along the western side of the Antarctic Peninsula. In addition, all sampled intrusive rocks are normally magnetized and are believed to have been emplaced during the Late Cretaceous period of predominantly normal polarity. There is no evidence of post-emplacement remagnetization. Unlike rocks from other Andean paleomagnetic collecting localities on the Antarctic Peninsula, whose mean declinations are oriented approximately north, the mean declination of samples from the Orville Coast and eastern Ellsworth Land is rotated 51° clockwise from north. Uncertainty in declination at the 95% confidence level (δ95) is ±27°. The data support the conclusion that the southern bend of the S-shaped Antarctic Peninsula was formed after Late Cretaceous time. Early Tertiary right-lateral transform faulting across the base of the Antarctic Peninsula may have produced this major oroclinal bend.Data from four localities (21 samples; I = −79°, D = 30°, α95 = 4.3°) in Upper Jurassic(?) massive rhyodacite porphyry lava flows in the northern part of the area are similar to those of the Andean igneous rocks. Although the evidence is not conclusive, it seems most probable that the porphyry was magnetically reset by Late Cretaceous plutonism. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleomagnetic results from the Lassiter Coast, Antarctica, and a test for oroclinal bending of the Antarctic Peninsula

Paleomagnetic results from 17 magnetically stable units of Upper Cretaceous (‘Andean’) plutons and dikes of the Lassiter Coast, on the southern Antarctic Peninsula, define a mean paleomagnetic pole at 87°S, 131°W (α95 = 11.5°). This indicates that little latitudinal movement of the southern Antarctic Peninsula has occurred during the past 100 m.y. All magnetically stable intrusives are normally polarized and are believed to have been emplaced during the Late Cretaceous epoch of predominantly normal polarity. There is no evidence of postemplacement remagnetization. The uncertainty in declination at the 95% confidence level is computed for both the Lassiter Coast data and those data available from other Andean sites in the Antarctic Peninsula. Within the limits of uncertainty, data from four localities north of 68°S support the contention of Dalziel et al. (1973) that there has not been any apparent post‐Late‐Cretaceous oroclinal bending in the northern half of the peninsula. For sites to the south in the Lassiter Coast, the uncertainty in declination, due to steep inclinations, is too large to support reliably or deny any large‐scale structural bending.

The management of sledge dogs in the Antarctic

The British Antarctic Survey (BAS) has relied on sledge dogs as the main form of transport for field parties since its predecessor, the Falkland Islands Dependencies Survey (FIDS), imported them in 1945, when it created a permanent presence on the continent. Over the years dogs have been used for extensive work over a very wide area. In 1975 BAS changed its policy with respect to work in the southern Antarctic Peninsula. In future seasons scientists will not winter in this area but will be flown to Adelaide Island and then into the field. They will carry out more detailed scientific work in particular areas of interest. This means that it is no longer practical to maintain huskies on base as they could not be kept fit and well disciplined during the winter for a short summer season. The scientists, similarly, could not be expected to learn to work the dogs efficiently in the time available, and the change in the nature of their work means that they will not move camp so frequently. The dogs, since they must be fed even if they are resting, would be uneconomic in such a situation. Finally, most of the work will be in areas that are thought to be safe so the dogs will not be needed to probe for crevasses. In future seasons BAS will rely on skidoos for field work (Figs 1 and 2).

Plate-tectonic model for southern Antarctic Peninsula and its relation to southern Andes

Three main Upper Jurassic–Cretaceous tectono-stratigraphic units recognized in Palmer Land and Alexander Island are comparable with assemblages formed (1) in an ensialic volcanic arc (Upper Jurassic Volcanic Group), (2) in a forearc or intra-arc marine basin (Fossil Bluff Formation), and (3) in a back-arc marine basin (Crabeater Point sediments and Latady Formation), possibly representing a proto–Weddell Sea. A sequence of intensely sheared, polyphasally deformed turbidites and pillow lava rocks of possible late Paleozoic or Triassic age, exposed along western Alexander Island, is tentatively interpreted as a trench assemblage accreted during an early stage of subduction. There is a close correlation between these units and the approximately coeval island-arc-marginal basin system of the southern Andes and South Georgia.